The next generation of solid-state lighting is seeking to provide advances in brightness, efficiency, color, purity, packaging, scalability, reliability and reduced costs. The creation of light emitting devices from silicon based materials, upon which the modern electronic industry is built, has been the subject of intensive research and development around the world. The main obstacle has been the indirect energy gap of bulk silicon, which limits the efficiency to an extremely low level. However, one particular technology, based on silicon nano-particles, e.g. nanocrystals, formed through various techniques, has been able to overcome this difficulty.
Prior art light emitting devices, such as those disclosed in United States Patent Publication Nos. 2004/149,353, entitled: “Doped Semiconductor Powder and Preparation Thereof”, published Aug. 5, 2004 in the name of Hill; 2004/151461, entitled: “Broadband optical pump source for optical amplifiers, planar optical amplifiers, planar optical circuits and planar optical lasers fabricated using group IV semiconductor nanocrystals”, published Aug. 5, 2004 in the name of Hill; 2004/214,362, entitled: “Doped semiconductor nanocrystal layers and preparation thereof”, published Oct. 28, 2004 in the name of Hill et al; and 2004/252,738, entitled: “Light emitting diodes and planar optical lasers using IV semiconductor nanocrystals”, published Dec. 16, 2004 in the name of Hill, which are incorporated herein by reference, have demonstrated that using silicon-rich silicon oxide (SRSO), which consists of silicon nano-particles embedded in a silicon dioxide (SiO2 or glass) matrix, reduces many of the problems associated with bulk silicon, and when doped with erbium, or other rare earth material, can exhibit efficient room temperature rare earth luminescence because of the high efficiency of the energy transfer process from excited nanocrystals to rare earth ions. Accordingly, the SRSO provides an alternative to thin film electroluminescent material. The silicon nano-particles act as classical sensitizer atoms that absorb incident photons or electrons and then transfer the energy to the rare earth ions, which then fluoresce in the infrared or visible wavelength ranges with several advantages compared to the direct fluorescence of the rare earth. First, the absorption cross-section of the silicon nano-particles is larger than that of the rare earth ions by more than three orders of magnitude. Second, as excitation occurs via an Auger-type interaction or via a Forster transfer process between carriers in the silicon nano-particles and rare earth ions, incident photons need not be in resonance with one of the narrow absorption bands of the rare earth. Unfortunately, existing approaches to developing such silicon nano-particle materials have only been successful at producing very low concentrations of the rare earth element, which is not sufficient for many practical applications.
Observations have shown that silicon nano-particles formed by such techniques generally have a relatively narrow distribution of photo-luminescent (PL) wavelength or energy despite the broad size distribution, i.e. the observed energies are not as high as expected from the quantum confinement of the nanocrystals. The reduced nano-particle excitation energy affects the efficiency of energy transfer from conducting electrons when films with embedded nano-particles are electrically powered, thereby causing severe limitations on the efficiency of light generating capability therefrom.
In general, the manufacture of group IV semiconductor nano-particles doped with a rare earth element is done by ion implantation of silicon ions into a silicon oxide layer, followed by high temperature annealing to grow the silicon nano-particles and to reduce the ion implantation damage. The implantation of silicon ions is followed by an ion implantation of the rare earth ions into the annealed silicon nanoparticle oxide layer. The resulting layer is again annealed to reduce the ion implant damage and to optically activate the rare earth ion.
There are several problems with this method:
i) it results in a decreased layer surface uniformity due to the ion implantation;
ii) it requires an expensive ion implantation step;
iii) it fails to achieve a uniform distribution of group IV semiconductor nano-particles and rare earth ions unless many implantation steps are carried out;
iv) it requires a balance between reducing the ion implant damage by thermal annealing while trying to maximize the optically active rare earth; and
v) the thickness of the film is limited because implanted ions do not penetrate deeply into the film for practical implant energies.
To diminish the above drawbacks, plasma enhanced chemical vapor deposition (PECVD) has been utilized to make group IV semiconductor nano-particle layers. The prepared layers are subjected to a rare earth ion implantation step, and a subsequent annealing cycle to form the group IV semiconductor nano-particles and to optically activate the rare earth ions that are doped in the nano-particle region. Unfortunately, the layers prepared with this method are still subjected to an implantation step, which results in poor surface uniformity, non-uniform distribution of rare earth elements, and limited film thickness.
Another deposition method that has been used to obtain a doped group IV semiconductor nano-particle layer consists of co-sputtering the group IV semiconductor and rare earth metal, typically in an oxygen plasma. In this method, the group IV semiconductor and the rare earth metal were placed on a target substrate, which was then placed into a vacuum chamber and exposed to an argon ion beam. The argon ion beam sputtered off the group IV semiconductor and the rare earth metal, both of which were deposited onto a receiving silicon wafer. The newly formed film on the silicon wafer was then annealed to grow the nano-particles and to optically activate the rare earth ions. The doped group IV semiconductor nano-particle layers made through this method have the drawbacks that: i) the layer does not have a very uniform distribution of nano-particles and rare earth ions; ii) the layer suffers from up conversion efficiency losses due to rare earth clustering in the film; and iii) the concentration of rare earth film in the film is limited to little more than 0.1%.
An object of the present invention is to overcome the shortcomings of the prior art by providing a carbon doped optically active layer in a solid state light emitting device.